660 HISTORY OF GEOLOGY. 



covered over by more horizontal fragmentary beds, 

 the opinion that some violent catastrophe had oc- 

 curred to dislocate them, before the superincumbent 

 strata were deposited, was strongly held. It was 

 conceived that a period of violent and destructive 

 action must have succeeded to one of repose ; and 

 that, for a time, some unusual and paroxysmal 

 forces must have been employed in elevating and 

 breaking the pre-existing strata, and wearing their 

 fragments into smooth pebbles, before nature sub- 

 sided into a new age of tranquillity and vitality. In 

 like manner Cuvier, from the alternations of fresh- 

 water and salt-water species in the strata of Paris, 

 collected the opinion of a series of great revolutions, 

 in which "the thread of induction was broken." 

 Deluc and others, to whom we owe the first steps 

 in geological dynamics, attempted carefully to dis- 

 tinguish between causes now in action, and those 

 which have ceased to act; in which latter class 

 they reckoned the causes which have elevated the 

 existing continents. This distinction was assented 

 to by many succeeding geologists. The forces which 

 have raised into the clouds the vast chains of the 

 Pyrenees, the Alps, the Andes, must have been, it 

 was deemed, something very different from any 

 agencies now operating. 



This opinion was further confirmed by the ap- 

 pearance of a complete change in the forms of 

 animal and vegetable life, in passing from one 

 formation to another. The species of which the 



